In a search for superior new anticancer drugs, we propose to synthesize various 3-furanosyl-5-aminopyrazolo (4,3-d)pyrimidine-7(6H)-thiones as the first thioguanine-type C-nucleosides. These products can be predicted to retain the antitumor properties common to thioguanine nucleosides. As C-nucleosides of the formycin class, they can be expected to mimic their purine analogs excellently in enzymatic processes, but will be inherently resistant to nucleoside cleavage. Thus, the proposed targets will not show the limitations of thioguanine nucleosides, which undergo cleavage to thioguanine, leading to toxic side effects and tumor resistance. This is a proposal in drug development for cancer treatment we believe has a high probability of success. Multistep syntheses are proposed, but efficient completion can be anticipated based on the extensive background for synthesis in the thioguanosine and formycin areas. Proposed as the sugar unit are beta-D-ribose, alpha- and beta-2-deoxy-D-ribose, and analogous 3-branched sugars suggested by another study of structure-activity relationships in the thioguanosine area.